Most tree diseases are transmitted using either a soil/root avenue or a wind/insect avenue. Some diseases can travel using either of these methods.
Insects and wind can carry many tree diseases from tree to tree (air-borne) if the condition of the tree provides a comfortable home for the pathogen. Soil-borne organisms move pathogens through the soil (water-borne), taken up by tree roots and spread by root grafts to other trees.
To prevent and control these diseases, you first have to identify the disease, then take measures to limit or remove the elements that provide disease transmission.
American Chestnut Blight, USFS/FIDL Photo